Photos may help us assess the frog. You might check your local area for an amphibian, or turtle, or more general wildlife rehabilitation centre willing to take care of it.
Euthanasia is an option. If it's a frog I hit with my mower that's my approach if the wounds are too severe to recover from and with a mower encounter this is almost always the case.
If the injuries weren't caused by man (i.e. an unsuccessful snake attack), letting 'nature take it's course' is another option. It may seem cruel, but imo it's better an injured frog returns to the food chain than a predator having to go after a healthy one.
On the belly is suggested, the pelvic patch is made to absorb so it will take it up quicker. More info on the unpleasant deed here http://www.frogforum.net/tree-frogs/...tml#post124199 or here Caudata Culture Articles - Euthanasia for Amphibians